Such a circuit arrangement having a load transistor M and a generally known voltage limiting circuit 10 functioning according to the principle of “active zenering” is illustrated in FIG. 1. In the example, the load transistor M is designed as an n-type MOSFET that serves to connect a load Z to a supply voltage according to an input signal Sin present at a drive input IN. A driver circuit DRV serves for converting the drive signal Sin to suitable levels for driving the transistor M. The drain-source path D-S of the transistor, which forms the load path thereof, is connected in series with the load between supply voltage terminals K1, K2 for the purpose of switching the load, one of which supply voltage terminals is at a supply potential V+ and the other of which supply voltage terminals is at reference-ground potential GND. In order to simplify the explanation, it is assumed hereafter that the reference-ground potential is ground, so that the value of the supply potential V+ corresponds to the value of the supply voltage.
The voltage limiting circuit 10 comprises, by way of example, a series circuit formed by at least one zener diode Z1 and a diode D1 connected oppositely to one another, so that there is always one of the components Z1, D1 operated in the reverse direction. This series circuit is connected between the drain terminal D and the gate terminal G of the transistor M.
Referring to FIG. 2, the limiting circuit 10 may also comprise a transistor T1, the load path of which is connected between the drain terminal D and the source terminal S of the load transistor. This transistor T1 is driven by a series circuit formed by a zener diode Z1 and a resistor R1, which series circuit is connected in parallel with the load path D-S of the load transistor M. In this case, the transistor T1 is connected to a node common to the zener diode Z1 and the resistor and is turned on if the drain-source voltage of the load transistor exceeds the breakdown voltage of the zener diode Z1 and the zener diode Z1 is thus turned on.
The voltage limiting circuit or protective circuit 10 connected between the drain terminal D and the gate terminal G of the transistor M protects the load transistor M in the off state from overvoltages by virtue of the limiting circuit 10 turning the transistor M on as soon as the drain-source voltage thereof reaches a predetermined maximum value. This maximum value to which the drain-source voltage of the transistor M is clamped by the protective circuit 10 is essentially determined by the breakdown voltage of the zener diode Z1 in both cases explained above.
Circuits corresponding to the limiting circuit 10 which protect the transistor M from overvoltages are used in a targeted manner in connection with the switching of inductive loads for the purpose of commutating the inductive load Z after the transistor M has turned off. After the presence of a turn-off signal at the drive terminal IN, and thus at the gate terminal of the transistor M, and in the event of the drain-source voltage rising, the limiting circuit 10 in this case holds the transistor M in the on state until the load Z has commutated to an extent such that the load path voltage of the transistor M has fallen below the value of the clamping voltage. During this operating state, in which the overall circuit with the limiting circuit 10 and the transistor M functions in the manner of a zener diode, the energy previously stored in the inductive load Z is converted into heat in the transistor.
Cellularly constructed MOS transistors having a multiplicity of transistor cells that are constructed identically and driven jointly are employed as load transistors in circuits in accordance with FIGS. 1 and 2. Such transistors of recent design, which are optimized in respect of having a low on resistance, may tend toward so-called current splitting in the case of low load currents such as may occur during commutation of inductive loads by means of limiting circuits according to the active zenering operation. This means that the load current is accepted only by a few transistor cells instead of being distributed uniformly over all the cells. In this case, destruction of individual cells and thus destruction of the entire transistor component may occur even though the maximum current-carrying capacity of the component is still far from having been reached. This problem is aggravated as the voltage present across the transistor increases and thus as the supply voltage increases. The supply voltage may rise in particular when a plurality of loads are connected to the supply voltage—like for example in a motor vehicle in which a multiplicity of different loads are connected to the vehicle battery—and when some of the loads are suddenly isolated from the voltage supply. In this context, the jargon talks of a sudden load drop (load dump).